Let's not argue about which of its elements is the defining one. Even if you haven't played them, the Age Of Empires games were a pillar of RTS design, and probably the most popular candidate for a comeback since publishers largely abandoned the genre in the 2000s. It's tempting to bang on about the history of Age Of Empires, but let's be real here, you already know it well enough. From: Steam, Microsoft Store / Xbox Game Pass.
It could still be friendlier to newcomers, but Age Of Empires 4 is an assured step in a genre whose comeback is long overdue.